This disclosure generally relates to compounds of formula I, including their salts, as well as compositions and methods of using the compounds. The compounds are prodrugs for ligands, agonists, and partial agonists for the nicotinic α7 receptor and may be useful for the treatment of various disorders of the central nervous system, especially affective and neurodegenerative disorders.
Schizophrenia is a serious mental disorder, affecting approximately 1% of the population. Its progressive course results in major impairment of mental and social functioning and often leads to the development of other pathologies. Susceptibility often runs in families, with both genetic and environmental factors thought to be important. The direct and indirect costs of the disease are estimated in the tens of billion dollars annually in the U.S. alone.
Patients with schizophrenia have an elevated risk of suicide (approximately a 10% lifetime risk). They have a 2.5 fold increase in all-cause mortality, resulting in a 20% lowered life expectancy. The onset of illness can result in cascade of unhealthy lifestyle factors and behaviors that elevate the risk of various conditions and consequently the risk of death.
The onset of schizophrenia is most often in late adolescence or early adulthood, and episodes recur throughout life. The disease is characterized by the expression of three distinct symptom domains: positive, negative and cognitive. Psychotic or positive symptoms include delusions, hallucinations, thought disorder and paranoia. Negative symptoms include negative affect, social withdrawal, and anhedonia. Cognitive dysfunction includes deficits in attention, working memory and executive function. The pathophysiology of schizophrenia is not well understood; however, most experts believe it is a multi-factorial disorder in which biological, genetic and environmental factors play a role. Most current therapies target the dopaminergic system and have resulted in the suggestion that an excess of dopaminergic neurotransmission underlies at least some aspects of schizophrenia. This theory received further support from findings that drugs which increase the levels of dopamine cause psychoses similar to the positive symptoms of the disease. Also, post mortem analysis of brains from schizophrenic patients indicate increased numbers of D2 dopamine receptors. Although newer antipsychotic agents, known as atypical antipsychotics, which are active at several additional neurotransmitter receptors, have been introduced in the past decade, these agents still share efficacy against the D2 dopamine receptor. All currently-used agents also have major limitations. Although positive symptoms are generally reduced in a majority of patients, these drugs do little to relieve the negative symptoms and cognitive deficits that are common and often most debilitating. In addition, antipsychotic agents have a number of unwanted and limiting side effects.
Nicotine is among the few agents which have a positive effect on cognitive function. Many schizophrenics smoke; the rate in patients is 2-4 times that of the general population, and up to 90% in schizophrenics who have been institutionalized do smoke. This smoking habit has been characterized as a form of self-medication.
Nicotinic acetylcholine receptors (nAChR's) are pentameric ligand-gated ion channels which are widely expressed through the central and peripheral nervous system. These channels are fast-desensitizing calcium channels which, when open, increase the intracellular concentration of the Ca++ ion. Although there are 12 individual receptors, the most abundant nicotinic receptors in the brain are α4β2 and α7. The α4β2 complex has been identified as the “high affinity” nicotine site. The homo-pentameric α7 receptor selectively binds the natural product, α-bungarotoxin, which has allowed its relatively facile localization and measurement. The α7 receptor is primarily expressed in the cortex, hippocampus and subcortical limbic regions and commonly occurs pre-synaptically. The localization of α7 nAChRs in areas involved with learning and memory has led to studies using both knockout mice and pharmacological manipulation. It is involved in sensory gating, memory, and neuronal plasticity. Alpha7 agonists have been shown to increase the release of neurotransmitters in rodents, including dopamine, serotonin, glutamate and GABA. Compounds which selectively bind to the α7 receptor, such as α7 agonists and partial agonists, have been shown to improve learning and memory functions in normal and aged animals, reverse scopolamine-induced memory deficits, reverse deficits in cognition induced by NMDA antagonists, reverse pharmacologically-induced gating deficits, e.g. amphetamine induced gating disruption, and to possess some anxiolytic properties. The α7 agonists of the present invention are expected to be useful in the treatment of schizophrenia and cognitive disorders associated with schizophrenia.
Alzheimer's disease is a progressive neurodegenerative disorder, resulting in the general loss of cognitive functions. The incidence increases with age, to the degree that 25-50% of all individuals over 85 are estimated to suffer from some degree of dementia. A diagnosis of Alzheimer's implies that the remaining life expectancy is reduced by half, compared to normal adults.
Clinical signs of Alzheimer's disease are progressive cognitive deterioration, decreased ability to perform the activities of daily living and neuropsychiatric symptoms or behavioral changes. In the advanced stages of the disease, deterioration of musculature and mobility may lead to inability to feed oneself, and eventually to the patient becoming bedridden. Language becomes severely disorganized, and then is lost altogether. Patients are not able to perform even simple tasks independently and require constant supervision. The cost of institutional care makes up nearly 70% of the cost of the disease. Therefore, therapies which increase cognitive function and delay institutionalization are greatly needed.
Alzheimer's disease has been shown in several studies to be accompanied by a reduction in nicotinic receptors in the cortex and hippocampus. Nicotine injections or nicotine skin patches have been reported to significantly improve attention, memory and learning in Alzheimer's disease patients. While there is a progressive loss of nicotinic receptors during the course of Alzheimer's disease, the α7 neurons are relatively spared, compared to the more abundant α4 receptors. Recently, the administration of selective nicotinic α7 agonists has been shown to increase cognitive functioning in Alzheimer's patients when dosed as long as 8 weeks. This clinical data is consistent with pre-clinical data showing α7 agonists and partial agonists improve learning and memory functions in normal and aged animals and reverse scopolamine-induced memory deficits. Thus, the compounds of the present invention may be useful in the treatment and prevention of Alzheimer's disease. The amyloid peptide Aβ42 has been shown to bind to the α7 nicotinic receptor (Wang et al., J. Biol. Chem., 2000, 275:5626-5632; J. Neurochem. 2000, 75:1155-1161). This association may facilitate the aggregation of Aβ42, believed to be important in the toxic effects of Aβ42, and may also cause disregulation of signaling through α7 nicotinic receptors. Deletion of the α7 receptor gene improves cognitive deficits and synaptic pathology in a mouse model of Alzheimer's disease (Dziewczapolski et al., J. Neuroscience, 2009, pp 8805-8815). The compounds of the present invention may disrupt the interaction of Aβ42 and α7 receptors. Treatment with α7 agonists and partial agonists may represent an approach for disease modification in Alzheimer's disease. Alpha7 receptors may also mediate inflammatory processes in neurodegenerative conditions, such as Alzheimer's disease (Conejero-Goldberg et al., Neurosci. and Biobehav. Rev., 2008, 32, pp 693-706). The α7 agonists and partial agonists of the present invention may be useful in reducing inflammation in neurodegenerative diseases and disorders, such as Alzheimer's disease.
The α7 receptor has also been shown to be involved in the reduction of inflammation via the vagus nerve. In addition, the α7 receptor is expressed in synoviocytes from RA and OA patients, and α7 agonists have been shown to inhibit the proinflammatory cascade that occurs in the rheumatoid joint (Waldberger et al., Arthritis and Rheumatism, Vol 58, pp 3439-3449). Thus, the compounds of the present invention may be useful in the treatment of inflammatory conditions, such as rheumatoid arthritis and osteoarthritis.
Nicotinic receptors containing the α7 subunit are present on mucosal mast cells known to be involved in gastrointestinal hypersensitivity (Kageyama-Yahara et al., Biochem and Biophys. Research Commun., 2008, v. 377, pp 321-325). The α7 agonist GTS-21 inhibits the antigen-induced degranulation of mucosal mast cells, suggesting that α7 agonists may be useful in the treatment of hypersensitive bowel conditions, such as ulcerative colitis.
In a recent report (Marrero et al., JPET Fast Forward, Sep. 28, 2009, DOI: 10.1124/jpet.109.154633), an α7 agonist was shown to decrease weight gain and food intake and reduce the elevated plasma levels of triglycerides, glucose, glycated hemoglobin and TNFα in a mouse model of type II diabetes (db/db mice which are deficit in leptin receptors). The α7 agonists and partial agonists of the present invention may be useful in the treatment of diabetes.
The following references provide general reviews of the nicotinic receptor system and α7 receptors and ligands: Picciotto and Zoli, J. Neurobio. (2002) 53:641-655; Brening, et al, Ann. Reports in Med. Chem. (2005) 40:3-16; Dani and Bertrand, Ann. Rev. Pharm. Tox. (2007) 47:699-729; Olincy and Stevens, Biochem. Pharmacol. (2007) 74:1192-1201; Broad, et al, Drugs Future (2007) 32 (2):161-70; de Jonge and Ulloa, Brit. J. Pharmacol. (2007) 151:915-929; Romanelli, et al, ChemMedChem (2007) 2(6):746-767; Lightfoot et al., Progress in Medicinal Chemistry (2008), v 46, pp 131-171; Concotta et al., Current Opinion in Investigational Drugs (2008), v 9, pp 47-56; Leiser et al., Pharmacol. and Therapeutics (2009), doi:10:1016/j.pharmthera.2009.03.009).
Ligands for the nicotinic α7 receptor have been disclosed. See: U.S. Pat. No. 7,863,291 and US 20100099684.
The invention provides technical advantages, for example, the compounds are novel and are prodrugs for ligands for the nicotinic α7 receptor and may be useful for the treatment of various disorders of the central nervous system, especially affective and neurodegenerative disorders. Additionally, the compounds provide advantages for pharmaceutical uses, for example, with regard to one or more of their mechanism of action, binding, inhibition efficacy, target selectivity, solubility, safety profiles, or bioavailability.